1. Field of the Invention
The present invention relates to hard disk drives and a method for determining a point of contact between a head and a disk for a head with a heater coil.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
The disks are rotated by a spindle motor of the drive. Rotation of the disks creates an air flow within the disk drive. Each head has an air bearing surface that cooperates with the air flow to create an air bearing between the head and the adjacent disk surface. The air bearing eliminates or minimizes the mechanical wear between the head and the disk. The height of the air bearing is commonly referred to as the flying height of the head.
The magnetic field detected by the head is inversely proportional to the flying height of the head. Likewise, the strength of the magnetic field written onto the disk is also inversely proportional to the fly height. A larger fly height will produce a weaker magnetic field on the disk.
Due to various reasons the flying height of the heads may vary during the operation of the drive. Such a variation in fly height may result in poorly written data on the disk. For example, weakly written data may create errors during a read routine. It is desirable to accurately determine and control the flying height of the heads in a disk drive.
There have been developed heads which include a heater coil. Current is provided to the heater coil to generate heat and thermally expand the head to move the read and write elements closer to the disk. These types of heads are sometimes referred to as fly on demand (“FOD”) heads. The flying height of FOD heads can be varied by changing the amount of power provided to the heater coil.
It is desirable to create a flying height that is nearly zero. To achieve a minimum flying height it is desirable to know the point of contact between the head and the disk. Attempts to determine head contact have included monitoring the position error signal (“PES”) of a servo routine in the drive. The results of such an approach are shown in FIG. 1. Such a curve has numerous maximis and minimis. Consequently, it is difficult to determine the point where the head makes contact with the disk.